In a light-water reactor for a nuclear power plant, a reactor pressure vessel housing a core fuel is contained in a containment vessel in order to prevent outside leakage of radioactivity in the event of a nuclear accident. There could be a case for some reasons where water supply to the reactor pressure vessel stops, or piping connected to in the reactor pressure vessel breaks, leading to a loss of coolant therein. Even in such a case, multiply-provided emergency core cooling systems are enabled to automatically supply coolant to the reactor pressure vessel to fully cool down the core without damaging core fuels, thereby preventing a severe accident before happens.
Actually, also in such a light-water reactor, the overheating or melting of the core fuel is assumed although it is stochastically a very rare or unlikely case such that the above-mentioned multiply-provided emergency core cooling systems loss their functions.
If the systems deteriorate into such a grave crisis, a molten corium moves to a lower plenum of the reactor pressure vessel to settle thereon. If such a crisis persists for a long time, the lower head of the reactor pressure vessel melts to be penetrated so that the molten corium outflows into the containment vessel to settle on a containment vessel floor. The molten corium settled on the containment vessel floor melts and erodes a liner or a concrete included in the containment vessel, thereby leading to a risk of breaking boundaries of the containment vessel. In addition, noncondensable gases such as carbon dioxide and hydrogen, etc. are formed by the reaction of the molten corium with the concrete, and pressurize the inside of the containment vessel, thereby leading to a risk of breaking the boundaries thereof. A means to reduce the risks has been proposed which leads cooling water to an area of the containment vessel holding the molten corium to cool down the corium, thereby inhibiting the reaction of the corium with the concrete thereof.
At least, a drain sump is set up on the containment vessel floor to which the molten corium outflows from a reactor pressure vessel. The drain sump collects leakage water to possibly leak during plant operation, and detects water leakage. The water collected into the sump is pumped outside the containment vessel via piping using a pump on a top lid of the sump.
In the unlikely event of a reactor severe accident involving outflow of the molten corium to the outside of the containment vessel, the corium spreads on the containment vessel floor to possibly inflow and settle inside the drain sump. A drain water transport pump is mounted on a top lid of the drain sump, and water suction piping is connected to the pump from the inside of the drain sump. The pump and the piping could contact the molten corium which travels down from the pressure vessel. For this reason, if a water suction system melts to break owing to the molten corium traveling down, the corium easily flows into the sump. In this case, the molten corium in the sump will be substantially thicker than that in the containment vessel floor. This reduces capability of cooling down the molten corium, thereby leading to a risk of making it difficult to protect the containment vessel.
Then, an idea is proposed which covers the upper side of a drain sump with a protective barrier having a structure. The structure enables it to collect leakage water into the drain sump, but prevents the molten corium from flowing into the drain sump.
FIG. 7 is a sectional view showing a drain sump of a conventional reactor containment vessel proposed previously. In FIG. 7, a drain sump 103 is provided to a containment vessel floor 101 in contact with a pedestal wall 102, and the upper part of the drain sump is covered with a corium shield 107. The corium shield 107 includes upper and lower walls 104, 105 which extend vertically upwardly and vertically downwardly from the containment vessel floor, respectively, and further an upper roof 106. The upper wall 104 includes two or more flow paths 108 passing therethrough. The paths 108 have bottom faces having the same level as the containment vessel floor, and are separated from each other. The form of the flow paths is designed so that leakage water is led into the drain sump 103 and the molten corium 109 solidifies inside the flow paths 108 not to settle in the drain sump 103.
In the above-mentioned conventional drain sump, the corium shield 107 serves as a means to prevent the molten corium 109 from flowing into the drain sump 103, while it is necessary to make comparably larger the height and thickness of the upper wall 104 in order to solidify the molten corium in the flow paths 108 provided to the upside of the upper wall 104. Consequently, the area of the drain sump is taken from that of the containment vessel floor 101, thereby reducing a spreading area for the molten corium 109 settling on the containment vessel floor 101. That is, the whole area of the containment vessel floor 101 cannot be used fully to cool down the corium, because the effective area of the containment vessel floor 101 is reduced not only by the area of the drain sump 3 but also by the area of the corium shield 107. This situation makes it difficult to efficiently cool down the molten corium 109. Moreover, the molten corium 109 could not solidify inside the flow paths 108 formed in the upper wall 104 to flow into the drain sump 103.